1. Field of the Invention
The present invention relates to a flat color display screen, and more specifically to so-called "cathodoluminescence" screens, the anode of which carries phosphor elements likely to be energized by electron bombarding. This electron bombarding can come from microtips, from layers of low extraction potential or from a thermo-ionic source.
2. Discussion of the Related Art
To simplify the present description, only color microtip screens will be considered hereafter, but it should be noted that the present invention relates, generally, to the various types of above-mentioned screens and the like.
FIG. 1 very schematically shows the conventional structure of a flat color microtip display screen of the so-called "switched-anode" type.
Such a microtip screen is essentially formed of a cathode 1 with microtips 2 and of a grid 3 provided with holes 4 corresponding to the locations of microtips 2. The cathode/grid is placed opposite to a cathodoluminescent anode 5, from which it is separated by a vacuum space 12, and a glass substrate 6 of which forms the screen surface.
Cathode 1 is organized in columns and is formed, on a glass substrate 10, of cathode conductors organized in meshes from a conductive layer. Microtips 2 are implemented on a resistive layer 11 deposited on the cathode conductors and are disposed inside the meshes defined by the cathode conductors. FIG. 1 partially shows the inside of a mesh and the cathode conductors do not appear on the drawing. Cathode 1 is associated with grid 3 organized in lines. The intersection of a line of grid 3 and of a column of cathode 1 defines a pixel.
Anode 5 is generally provided with alternate bands of phosphor elements 7r, 7g, 7b, each corresponding to a color (Red, Green, Blue). The bands are parallel to the cathode columns and are separated from one another by an insulator 8. Phosphor elements 7 are deposited on electrodes 9 formed of corresponding bands of a transparent conductive layer such as indium and tin oxide (ITO). The sets of red, green, blue, bands are alternatively polarized with respect to cathode 1, so that the electrons extracted from the microtips 2 of a pixel by the electric field created between cathode 1 and grid 3 are alternatively attracted by the phosphor elements 7 of each of the colors.
To increase the brightness of the screen, it is desirable to increase the anode-cathode voltage and thus the anode-cathode distance defined by space 12. This raises two particular problems. The first one is that it is difficult, with existing components, to rapidly switch high voltages, and that this results in high power consumption. The second one is that the larger the inter-electrode space, the more the electrons emitted by the microtips scatter and tend to cause a parasitic illumination of the pixels neighboring that which is desired to be illuminated.
To solve the first problem, so-called "unswitched-anode" screens, the anode of which is formed of phosphor elements of different colors, all simultaneously polarized to a same potential, can be used. In this type of screen, the selection of the color of the phosphor element to be energized is obtained, on the cathode side, by dividing each column of the cathode into three sub-columns respectively associated with each of the colors.
However, the second problem of parasitic illumination remains. This phenomenon is even more critical in this case since the electrons also bombard phosphor elements of another color.